Several types of microwave diagnostics, in the category of electron cyclotron emission (ECE) spectroscopy and reflectometry, have been developed on the Large Helical Device (LHD). Since LHD has a complicated magnetic configuration, the polarization effects have been studied for optimization of the microwave passive and active diagnostics. It was found that if the density is sufficiently high, the effect of mode conversion is negligible and the local polarization angle can be estimated as the angle at the plasma boundary. Three types of ECE spectroscopy, which are the heterodyne radiometer, the Michelson spectrometer, and the grating polychromator, have been optimized and operated routinely in order to measure radial profiles of electron temperature and its fluctuations in the frequency range 50 to 500 GHz. Several types of microwave reflectometers have also been utilized for measurements of the electron density profile and fluctuations. Two ultrashort pulsed radar reflectometers for density profile measurements, a V-band frequency-hopping reflectometer for density fluctuation profile measurements, and a fixedfrequency three-channel homodyne reflectometer for the interlock system of the neutral beam injection have been routinely operated. Also, an advanced diagnostic, which uses an imaging technique, has been developed to study the two- or three-dimensional structure of temperature and density fluctuations.